Field of the Invention
In general the invention relates to apparatuses and processes used for the reduction of metal oxides and in particular to apparatuses and processes which employ electric plasma torches to supply the necessary thermal energy for the reduction reaction.
The production of iron and ferroalloys by carbothermic reduction is generally accomplished using blast furnaces, direct-reduction shaft furnaces, or electric smelting furnaces. The blast furnace traditionally used for iron making consists of a downwardly moving coke/iron ore bed within a vertical shaft. Preheated air is admitted at the bottom of the shaft traveling countercurrent to the bed while reacting with the coke to provide both carbon monoxide for reductant and the required heat to satisfy the process energy requirements. The blast furnace is very well optimized in terms of scale economics, energy efficiency and material yield. It has, however, several inherent shortcomings. These include high coke consumption per ton of iron produced; agglomeration of the ore prior to processing; economic operation only at very large capacities, e.g., over 2 million tons/year; the product iron is fully saturated with carbon (i.e., carbon content is greater than 4%); high gas flow rates and resultant pollution control equipment for the cleaning of the gas; and high capital costs. The cost of iron production using state-of-the-art blast furnaces is on the order of $200/ton of iron produced.
As an altenative to the blast furnace, direct reduction and direct electrical smelting techniques evolved. The former produces metallized pellets suitable for melting in an electric arc furnace. Prior to direct reduction, iron oxide is pelletized then indurated by firing at a temperature of 2400.degree.-2500.degree. F. The pelletizing process adds about $30-$40 in costs per ton of iron due to the large amount of energy required for firing of the pellets. The pellets are fed into the top of a direct reduction shaft furnace where they move slowly downward against the upward flow of reducing gases having temperatures of about 850.degree. C. In the pellets, metallization levels of 92-94% are typically achieved under industrial operating conditions. Upon leaving the reduction shaft, the reduction gas is regenerated and recycled back to the reduction furnace. The pellets are then fed into the electric arc furnace wherein an electric arc is used for melting and alloying of the pellets into steel products. With this system lower capital costs are achieved. However, the rapid escalation of the price of natural gas, which is used as the source of the reducing gases in the reduction furnace, has made this approach somewhat uneconomical.
Development of a process for cost competitive iron making should include the following features. The source of iron units should be iron ore concentrate, not pellets or sinter. The principal carbon source for the reduction reaction should be coal rather than the more costly coke, oil, or natural gas. The product should also be molten iron directly useful for steel making or capable of shipment without further treatment to prevent reoxidation. Further, the carbon content should be controllable at levels in the range of 0.1 to 4.0%, thus the product could be used as pig iron or could be directly useful for steel making. Lastly, the energy requirement should be minimized.
Processes having some of these features have been patented in various configurations. Westinghouse developed a system for the plasma smelting of prereduced ore, incorporating fluid beds for preheating and prereduction of ore concentrate using the off gases leaving the plasma reactor. This process is described in U.S. Pat. No. 4,061,492 entitled "Method of Ore Reduction with an Arc Heater" and issued Dec. 6, 1977 and assigned to the assignee of the present invention. The Boliden process described in U.S. Pat. No. 3,365,185 entitled "Production of Metals from Pulverulent Materials by Flash Melting in an Electrically Heated Furnace" and issued Jan. 23, 1968 and U.S. Pat. No. 3,563,726 entitled "Production of Metal from Pulverent Material by Flash Smelting in a Vortex" and issued Feb. 16, 1971 utilizes the flash smelting of ore in an electrically heated furnace. There, oxygen, pulverized coal, and iron ore concentrate are combined in a flash smelter to provide a molten partially reduced matte which is reduced to iron after forming a slag layer in a submerged arc furnace located below the smelter. Off gases from the smelter are burned and recuperated in a steam generator to provide the arc furnace power. The ASEA process trade-named ELRED employs an electric arc furnace for the final smelting of the iron ore in a series of countercurrent fluid beds for the prereduction of the incoming ore concentrate by contact with the hot reducing gases leaving the furnace. A further process described in U.S. Pat. No. 4,072,504 entitled "Method of Producing Metal from Metal Oxides" and issued Feb. 7, 1978, employs electric arc or plasma torches to fire the bottom of a coke filled shaft furnace. Prereduced iron ore concentrate and pulverized coal are injected into the plasma-heated gas stream at the bottom of the coke-filled shaft furnace where smelting is accomplished. The gases leaving the furnace are used to preheat and prereduce the iron ore in countercurrent fluid beds similar to the Westinghouse and ASEA systems In U.S. Pat. No. 4,425,659 entitled "Metal Oxide Reduction Furnace" and issued Jan. 10, 1984, an electrode mounted through the side of the lower portion of the furnace generates an arc that burns a cavity in the coke column contained therein with a stream of metal oxide particles mixed with a reducing agent is injected into the cavity where the reduction reaction occurs. The coke column extending directly above the reaction zone filters contaminants such as liquid hydrocarbon or coal from the reaction gases rising through the coke column. The coke column also serves to shield the reactor walls from the arc flare occurring at the lower portion of the reactor. In U.S. Pat. No. 3,834,895 entitled "Method of Reclaiming Iron from Ferrous Dust" and issued Sept. 19, 1974 iron bearing particulate waste is introduced into a plasma-arc furnace with the molten iron being collected at the bottom for discharge into ingots or directly into a steel making furnace.
All of the cited art teach the principles of smelting combined with prereduction and thus all feature the same energy requirements of about 3 to 4 KGcal/ton of product iron. However, all have the same practical shortcoming when considered for use with U.S. Mesabi taconite iron ore or other metallic ores of very small particle size (10 microns to 1000 microns). Elutriation and carryover from the fluid beds or flash smelter reactor and the resultant loss of ore from the system occur. Although in the first two patents cited previously, the coke bed is used both for reaction surface as well as recovery of finely divided iron droplets. They both have the disadvantage of carbon saturating the molten iron as it contacts the matrix of coke at the bottom of the shaft. The carbon saturation is the result of a long contact time between the iron pool or bath and the coke matrix that occurs between tapping of the melt.
An apparatus and process in which the contact time of the metal with the coke matrix can be minimized would therefore be advantageous. Further, the ability to control the time of contact of the metal with the coke and thus be able to control the percentage of carbon dissolving into the molten metal would also be advantageous. Further, an apparatus and process useful for the direct reduction of one having a very small particle size and allowing control of the carbon content thereof would also be beneficial.